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The paper deals with the accurate slip measurement of induction motors. The proposed procedure uses a noninvasive slip measurement scheme based on digital filtering and dynamic parameter estimation. The slip measurement is carried out without speed sensor and is deduced analyzing the magnetic field harmonics spectrum in proximity of the induction motor. First the EMF induced waveform, taken from a searching coil, is filtered using algorithms based on the discrete Fourier transform. Then the stator frequency and rotor frequency are obtained by comparing the filtered voltage with a mathematical model using an optimization procedure. The model's parameters are varied until an adequate match is obtained with the filtered voltage. Experimental results are presented to validate this method.